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Custom ROMs have had just about enough of being Android's second-class citizens
(www.androidauthority.com)
This is a most excellent place for technology news and articles.
Huawei is being forced to do it. But like Android, their HarmonyOS is not 100% open-source. There's also KaiOS, which some Nokia and Alcatel, and all Jio, devices use.
Both the ones I mentioned are designed to be more memory efficient. KaiOS in particular is aimed primarily at feature phones and entry-level smartphones.